The present invention generally relates to sheets prepared from fiber, latex and a coalescing agent. Further, the present invention provides for a sheet having improved stiffness by employing a hard latex and a fugitive coalescing agent.
A wide variety of sheets made from fibrous materials have been described in the art. Typical examples of such sheets include fine printing papers, cardboard papers, underlayment felt for vinyl floor coverings, gasket papers, roofing papers, sound-deadening papers, pipewrap, heat deflection papers and board products.
In an effort to improve the properties of these products, various latex compositions have been employed. While such practices have greatly contributed to the art, it is still desirable to have further improvements in their physical properties such as stiffness.
Stiffness is very important in printing grades of paper, especially in the lightweight grades. Methods to increase sheet stiffness are therefore desirable. One potential method for increasing stiffness would be to employ a latex having greater hardness; however, use of such a latex would be limited by its ability to coalesce or form a film under the sheet preparation conditions. That is, a latex with a minimum film forming temperature greater than the temperature a sheet reaches during preparation would not be a viable choice. Unfortunately, such latexes could provide the desired stiffness but increasing the drying temperature would be harmful to the fiber and other organic components of the sheet system. Therefore, it would be desirable to be able to employ a latex having minimum film forming conditions greater than the sheet preparation conditions in order to take advantage of the latex's physical properties (i.e., hardness which could translate to increased stiffness when used in a sheet system).
U.S. Pat. No. 4,225,383 generally teaches composite sheet preparation and suggests polymers which are made film forming by the use of plasticizers can be used. This reference is, however, limited to composite sheets containing relatively high levels of filler or pigment from 60 to about 95 percent based on the dried weight of the composite sheet. The subject invention, instead, focuses on sheets with no filler or medium filler levels.